In the early hours of New Year's day, Eric Rivera stood in a train at a subway station, watching with horror as a transit police officer on the platform shot an unarmed man.

Amateur video posted on Reddit.com shows officer holding his head after a shot was heard. Reports say he may have mistaken his Sig Sauer semi-automatic handgun for a Taser X26 stun gun, which is smaller and less than half the weight. (1) At 1:26 minutes into the video, BART officer behind Oscar Grant (face down on floor) appears to have a weapon in his right hand. (2) At 1:29, officer looks down after a shot was heard. (3) At 1:31, he holds his head in two hands. Sig Sauer 0.40 semi-automatic (bottom centre): Length 18 cm, height 14 cm, weight 500 to 900 grams, depending on model. Taser X26 (bottom right): Length 15 cm, height 8 cm, weight 200 grams.

OAKLAND, Calif.–In the early hours of New Year's day, Eric Rivera stood in a train at a subway station, watching with horror as a transit police officer on the platform shot an unarmed man.

What Rivera can only speculate about is whether the officer intended to use his Taser instead of his .40-calibre handgun.

"What I don't know is whether that officer drew the wrong weapon," said Rivera, who was riding the Bay Area Rapid Transit (BART) train on New Year's ay and was on a train that had stopped in the Fruitvale station at the time Oscar Grant, 22, was fatally shot.

For days, cellphone and camera videos filmed by passersby have been circulating on television and the Internet. They show transit police officer, Johannes Mehserle, 27, straddling Grant from behind, pulling out his weapon and shooting Grant in the back.

Yet another video surfaced yesterday, a digitally superior version of the event, that shows Mehserle clearly looking stunned seconds after the shooting, as if just realizing what's happened.

The shooting has incited widespread anger toward the police in a city where there has been a long and an uneasy relationship between police and residents. The fury erupted Wednesday night as protestors stampeded through the city burning cars and smashing storefronts. Police clad in riot gear responded with tear gas and batons, arresting up to 120 people.

"I'm not paranoid but frankly I don't trust the cops," said Camari Williams, 20, who had known Grant since he was 9 years old. "I don't know anyone in my circle of friends and family who trust the cops. It's that bad. You mention the cops and everyone has a story."

In his last moments of life, Grant, the father of a 4-year-old daughter, struggled against being handcuffed and asked several times for police not to shoot. After amateur videos showing Grant's shooting began to circulate, Mehserle and his family were forced into hiding. They have received death threats and the city has been rent by violent protests.

Grant's family is suing BART for $25 million.

John Burris, the lawyer representing the family, said if the officer did not have a Taser, the shooting was intentional and the officer should be charged with second-degree murder.

However, "if he thought he was reaching for a Taser and pulled a gun, that's negligent. That would be involuntary manslaughter, a negligent homicide," Burris said.

BART police officers began carrying the Taser in early December.

Burris told reporters there was nothing on the video to suggest the officer was reaching for anything other than his handgun.

"They keep those Tasers on the opposite side of their gun hand, or in the middle, pointed the opposite direction so you have to turn your hand to get it."

Police trainer Dave Smith said there are clues in the video that Mehserle could have mistaken a Taser for his handgun.

Describing the transit officer's movements, Smith notes: "He's in non-traditional firing stance. His feet aren't squared, he's off-balance. He shot him exactly where he would have shot if it was a Taser. If I train you in one skill that involves thousands of repetitions, it's going to be a habit."

Smith suggests Mehserle, who was not used to a Taser, reached automatically for his handgun through force of habit.

Although the two weapons feel different, the mistake is not uncommon.

On Wednesday night, hours after it became public that Mehserle, a two-year veteran of the force and the father of a week-old baby girl, had resigned, emotions boiled over in the downtown riots.

Oakland Mayor Ron Dellums went out that night to calm residents, reminding them that only a few weeks ago they were celebrating the election of Barack Obama.

"Don't let these few weeks ... rip that hope from your chest," he said.

"Stay hopeful and we cannot be victims."

The next night, Oakland police were out in full riot gear breaking up small groups that gathered in the city's downtown.

The victim's mother, Wanda Johnson, pleaded for calm. "Oscar would not want to see the violence and the fires," she said. "I'm asking you please, please stop it and let justice prevail."

Keith Snodgrass, who rides the train daily from the Fruitvale station to his job in San Francisco, said he believes the protests are helping residents deal with long-standing frustrations.

Dressed in a suit jacket and lace-up shoes with a fedora, the clean-shaven Snodgrass said no matter how well he is dressed, he has been pulled over by police near the train station many times and questioned even though he's done nothing to draw attention to himself.

"There is an inner-city cop mentality that it's us against them. People of colour get harassed, get searched, more than white people," said Snodgrass, a union representative for the city of San Francisco. "It's like a volcano and there's an active community here who is getting the idea that they have to heat up in order to get changes."